1. Field of the Invention
This invention relates to ion mobility spectrometry, in particular to methods and devices for generating and delivering of ammonia gas as dopant into the ionization region of an ion mobility spectrometer.
2. Description of the Related Art
Ion mobility spectrometry is based on characterizing chemical substances by the gas-phase mobility of their ionic species under the influence of an electric field. It has been known as an analytical technique since the late 1960s and early 1970s. Ion mobility spectrometers (IMS) operated at ambient pressure are highly sensitive for detecting substances at low concentrations in ambient air and from vaporized samples and have been successfully utilized for the detection of environmental pollutants, explosives and illicit drugs in the civil sector as well as for the detection of chemical warfare agents (CWAs) in the military sector.
The drift-type IMS are most commonly used in commercial instruments and are based on following principles: a gas sample or vapor from a heated sample is introduced into an ionization region to form ions of the gas-phase substances. The ions are introduced into a drift region in a pulsed manner and migrate under the influence of a homogeneous static electric field through a drift tube, normally against a counter flow of dry carrier gas. An ion detector provided at the end of the drift tube is used to measure the drift times taken by the ionic species to pass through the drift tube. The ion mobility of the ionic species can be calculated from the measured drift times, length of the drift tube and the electric field strength in the drift tube. There are other types of IMS operated at ambient pressure, for example Differential Mobility Spectrometry (DMS, also known as Field Asymmetric Ion Mobility Spectrometry, FAIMS) and the aspiration-type IMS.
The gas-phase substances are introduced by a carrier gas into the ionization region of the IMS and are most commonly ionized by chemical ionization (CI). The carrier gas of an IMS is typically purified air with some parts per million (ppm) of water vapor. Electrons emitted from a radioactive beta emitter, such as 63Ni, generate positive nitrogen ions by electron impact ionization and negative oxygen ions by electron attachment of thermalized electrons. The nitrogen and oxygen ions further react with water molecules present as vapor in the carrier gas to generate positive (H+(H2O)n) or negative water cluster ions (O2−(H2O)n), respectively. These secondary reactant ions react with gas-phase substances by protonation forming positive product ions, or by adduct formation forming negative product ions. The primary oxygen ions may also react with gas-phase substances by de-protonation, electron transfer or adduct formation forming negative product ions. The electrons generating primary reactant ions can be provided by radioactive as well as by non-radioactive electron sources, such as those using corona discharges, electron beam generators and/or UV/X-Ray lamps.
It is well known in ion mobility spectrometry that an additional gas-phase reagent (dopant) can be provided in the ionization region together with the gas-phase substances to be analyzed in order to improve sensitivity and selectivity to substances of interest (target substances), or to improve the rejection of interfering substances (i.e., those substances which may otherwise give rise to a signal interfering with the ion signal of the target substance(s)). Dopants modify the composition of the reactant ions such that the composition of the product ions is changed to improve the sensitivity and selectivity of the IMS. In the positive mode (i.e., when positive product ions are measured in the IMS), it is necessary that the ionic species of the dopant (including molecular ions and adduct ions with water) have a proton affinity less than that of the substance of interest and higher than that of other interfering substances such that product ions of substances of interest are preferably generated compared to interfering substances. Furthermore, since the proton affinity of the substance of interest is greater than that of the dopant ions, the gas-phase substances of interest can almost completely react with the ionic species of the dopant to effectively form product ions of the substance of interest.
Furthermore, dopants typically shift peak positions of product ions in the mobility spectrum by formation of adducts such that the signal peaks of substances of interest can be separated from each other or from signal peaks of interfering substances and can thus be identified even in the presence of interfering substances. If no dopant is added, the signal peaks of substances of interest can be hardly distinguished due to peak overlap.
Dopant delivery devices for use in IMS commonly comprise a sealed dopant reservoir with a permeation capability containing a dopant material, with the dopant reservoir being incorporated in the gas circulating system comprising a pump and means for drying and cleaning the recirculating gases. The commonly used dopants for detecting drugs in the positive mode include acetone and preferably ammonia. Dopants usually used for detecting explosives in the negative mode are halogenated compounds, such as chloride. For CWAs, ammonia is preferably used as dopant in the positive mode whereas in the negative mode usually no dopant is used because many target substances contain halogens.
Some IMS have been known to deliver ammonia gas as dopant by evaporating liquid anhydrous ammonia. While operating the IMS with dopant delivery, a dopant reservoir filled with liquid ammonia facilitates a controlled release of ammonia gas into the carrier gas after permeation through a membrane located between the liquid ammonia and the gas circulating system of the IMS. However, the use of liquid ammonia creates a number of difficulties. Ammonia is considered a highly toxic material and, according to the International Air Transport Association, may not be transported on passenger aircraft. Prior to use in the IMS, the dopant reservoir is typically hermetically sealed and frozen. Liquid ammonia must be pressurized to maintain a liquid form at room temperature. The vapor pressure of ammonia at room temperature is already about 8,600 hPa and a dopant reservoir must therefore be able to withstand a pressure of approximately 14,000 hPa without leaking. Because such a dopant reservoir is highly pressurized while operating the IMS with dopant delivery, the size of the dopant reservoir and thus its operating time is limited and the transportation of the IMS may be further restricted.
U.S. Patent Publication 2002/0088936 A1 (by Breach et al.) describes an IMS with a gas circulating system comprising means for drying and/or cleaning the circulating gas and a dopant source, wherein said means and the dopant source are physically combined, obviating the need for a separate dopant source. The dopant source material may be combined with the material for drying and/or cleaning the circulating gas, for example a molecular sieve material filled with adsorbed ammonium gas.
U.S. Patent Publication 2009/0255351 A1 (by Stearns et al.) describes a device for delivering ammonia gas for use in an IMS without having a reservoir containing ammonia. The delivery device includes an ammonium solid (e.g., ammonium carbamate (NH2COONH4) or ammonium carbonate ((NH4)2CO3), that will, upon the introduction of heat, release ammonia gas for delivery into the IMS. The volumetric flow rate of the ammonia gas is controlled by the use of capillary tubes as the exiting pathway, where the flow rate is determined by the cross sectional area and length of the capillary tube. Delivery of the ammonia is aided by use of a frit or screen to permit only gas to exit.
U.S. Patent Publication 2009/0166531 A1 (by Reda) describes a device for delivering ammonia gas for use in an IMS. The device includes a gas permeable tube containing an ammonia compound and is sized to be inserted into a space within the IMS. The device is configured to activate the ammonia compound to decompose into an ammonia gas that does not include water vapor, and emit the ammonia gas into the IMS. In the exemplary embodiment, the ammonia compound is ammonium carbamate (NH2COONH4) which decomposes into ammonia gas NH3 and carbon dioxide CO2 without producing any water vapor.
U.S. Patent Publication 2011/0240838 A1 (by Debono et al.) describes a device for delivering ammonia gas for use in an IMS comprising a permeation tube, ammonium sulfate disposed within the permeation tube in solid form, and a heating device configured to heat the permeation tube so as to create ammonia gas.
It is an ongoing aim to provide an IMS with a dopant delivery device for generating ammonia gas wherein the device has low power consumption and delivers ammonia gas at a preferably constant flow rate over a wide range of operating temperatures. It is a further aim that the dopant delivery device should have an operating time not further limiting the operating time of the IMS.